CENTENNIAL --A gunman opened fire at Arapahoe High School on Friday, critically wounding one student.

The suspected gunman was later found dead from an apparent self-inflicted gunshot wound. Another student was injured in the attack, although authorities said it was unclear if it was from gunfire.

Police SWAT teams swarmed the school, and squads were called to the scene.

The shooting began about 12:30 p.m.

Parents of Arapahoe High School students were asked to go to Shepherd of the Hills church, at 7691 S. University Boulevard, to reunite with their children.

"I was scared and shaking," one student said. She said she heard, "bang, bang, bang" and by the third shot she was on the ground.

Students said they hid in the corners of dark classrooms until they were led out of classrooms. Adam Jones, whose sister goes to high school, said students are still locked in rooms about an hour after the shooting began and were "very scared."

A teacher wrote in a text message from inside the school: "lockdown-super scary."

Nich Herzog, a senior at Arapahoe High School, stayed home from school because he was sick. He said he learned about the shooting after getting messages from friends in other schools asking if he was ok.

Herzog then started texting his friends at the school to find out what was happening.

"A lot of them are scared," Herzog said. "One of my friends was right next to someone who got shot and she's really shaken up. They are really scared and they want to get out."

Arapahoe High School has approximately 2,100 students, according to a message posted on its website by principal Natalie Pramenko. In response to the shooting, all high schools in Littleton Public Schools, which includes Arapahoe High, are on lock down. Schools in Aurora and Douglas County were also locked down.

Parent Julie Kellogg was driving by Arapahoe High School when she saw police rush to the campus. Kellogg said she frantically began calling and texting her children at the school but did not hear back. She said she knows they are OK now but the situation was horrifying.

"The shooter started off in the front office and last I heard he was in the library," Kellogg said, describing what she heard from employees and students who were evacuating the campus. "It was actually the most frightening thing I've ever been through.

"I would have never expected my reaction to be what it was," Kellogg said. "I immediately went into panic, broke down. I didn't know what to think and I didn't even know what happened but it I knew it was bad. You could tell it was bad."

The manager of a Subway across the street from the high school said Friday afternoon that the area is swamped with police cars and ambulance and fire trucks. She wasn't aware of what was happening at the school. An employee at the King Soopers in the shopping center also said there were many students at the scene.

In this still image taken from video provided by Fox 31 Denver, students gather just outside of Arapahoe High School as police respond to reports of a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013. Colorado division of emergency management spokeswoman Micki Trost said her director went to the school and their weren't any more immediate details. (AP Photo/KDVR) MANDATORY CREDIT
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"We saw some kids running on the south side of our parking lot," a secretary with the school said.

Becky O'Guin, a spokeswoman with South Metro Fire Rescue, said firefighters were called out to the school at 12:30 p.m. on reports of a fire.

"We're on the scene with fire units and medics," O'Guin said.

The Federal Bureau of Investigation also had agents on scene.

Saturday is the one-year anniversary of the Sandy Hook school shooting in Connecticut.